When a project is upgraded you have to apply the correct license for the version that you are using. This means that you have to apply a valid license for version 7.3. If the previous version was 7.1 the license file located in ~\App_Data\Sitefinity
folded is valid only for the 7.1.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items